Method for the production of a mineral substrate with modified surface and substrate thus obtained

ABSTRACT

Provided is a method for the production of a mineral substrate with a surface modified by organic groups. The method comprises placing the surface of a mineral substrate with silanol functional groups in contact with a solution of an organotrihydrosilane in an organic solvent at a temperature of less than 30 ° C. The mineral substrate with silanol functions can comprise silica particles, a sheet of glass, quartz or mica as well as silicon of the wafer type covered by a layer of silica deposited by an appropriate preliminary treatment.

The present invention relates to a process for the production of aninorganic substrate which is surface-modified by organic groups, and themodified substrates obtained.

The use of coupling agents, which make it possible to improve adhesionbetween an organic matrix and an inorganic substrate by forming anintermediate film, is increasingly widespread. Self-assembledmonolayers, called SAMs, which are formed by aliphatic long-chainorganic molecules on a silica substrate, constitute an alternative tothe films formed by physisorption according to the Langmuir-Blodgetttechnique. These SAM monolayers possess great stability and resistanceto various disruptions, in particular to corrosion and to the presenceof solvents, because the organic molecules are attached to the silica bycovalent bonds.

Various techniques for grafting an organic layer onto the surface of asilica substrate are known: organization of the layer by physisorption,for example grafting of an alkane onto a gold or silver substrate,starting with alkanediols; organization of the layer by chemisorption,for example grafting of an alkane onto a platinum substrate startingwith alcohols or amines, or onto an alumina substrate starting withcarboxylic acid; grafting of organic groups onto a substrate containingsurface OH groups, by covalent bonding starting with organosilanes suchas alkylchlorosilanes, alkylalkoxysilanes or alkylaminosilanes (cf. inparticular A. Ulman, Chem. Rev., 1996, 96, 1533-1554). In a method forgrafting organic groups onto a silica substrate carrying Si—OH groups,starting with organotrichlorosilanes, hydrochloric acid is formed whichcatalyzes both the hydrolysis reaction which causes the attachment ofthe organosilane to the surface of this substrate, and thehomocondensation of organosilanes with each other. The overall processis thus accelerated at the expense of selectivity. In the case ofshort-chain organochlorosilanes, which are the silanes most widely usedin industrial applications, the deposits obtained are in the form ofmultilayers whose thickness is difficult to control. When anorganotrialkoxysilane is used, the corresponding alcohol, which canbecome adsorbed to the surface of the substrate, is formed, causing anincrease in the heterogeneity of the grafting.

Through E. Lukevics et al., (J. Organomet. Chem. 1984, 271, 307),processes are known which consist in reacting organosilanes withcompounds having an active hydrogen, such as acids, alcohols and thiols.This process requires, however, the use of a catalyst, for example aLewis base, or of a nucleophilic solvent.

Through A. Fadeev, et al., (J. Am. Chem. Soc. 1999, 121, 12184), aprocess is known which consists in reacting an organosilane RSiH₃,R₂SiH₂ or R₃SiH with titanium oxide. Through A. Ulman et al., (Chem.Mat. 2002, 14, 1778 ), a process is known which consists in reactingoctadecyltrihydrosilane with γ-Fe₂O₃ particles. The films obtainedaccording to these processes are not very stable because the reactionsresult in the formation of labile Si—O-M bonds (M being depending on thecase Ti or Fe), which can be redistributed as Si—O—Si+M-O-M which aremore stable.

The aim of the present invention is to provide a process for theproduction of silica substrates which are surface-modified by depositionof a homogeneous and well-organized dense layer.

The process according to the invention consists in bringing an inorganicsubstrate carrying silanol functional groups at its surface into contactwith a solution of an organotrihydrosilane in an organic solvent, at atemperature of less than 30° C.

As an example of an inorganic substrate carrying silanol functionalgroups at its surface, there may be mentioned in particular silicaparticles, glass plates, quartz plates or mica plates, and wafer-typesilicon coated with a silica layer deposited by an appropriatepreliminary treatment.

A wafer-type silicon substrate carrying a silica layer at its surfacemay be obtained according to various processes. A first process consistsin removing the native silica layer by immersing the silicon substratein a solution of HF containing at least 10% by volume of HF in ultrapurewater under ultrasound, in rinsing with ultrapure water, and then intreating with ozone under UV. Such a treatment, which is particularlypreferred, is described in particular by J. R. Vig, J. Vac. Sci.Technol., 1985, 3, 1027-1034. A second process consists in subjectingsaid silicon substrate to an oxygen stream at high temperature, forexample at 1150° C., as described in particular by D. L. Angst,Langmuir, 1991, 7, 2236-2242. In another process, the silicon substrateis subjected to a chemical oxidation by the basic route: after cleaningthe surface of the substrate with a solvent under ultrasound, thesubstrate is left in an H₂O, NH₄OH, H₂O₂ 5/1/1 mixture, and then rinsedwith deionized water, dried and rehydrated (cf. for example “J. D.Legrange, et al., Langmuir, 1993, 9, 1749-1753”). In another process,the silicon substrate is subjected to a chemical oxidation by the acidicroute: the substrate is cleaned with a basic solution, and then dippedin an acidic mixture of the H₂SO₄/H₂O₂ type (cf. A. K. Kakkar, et al.,Langmuir, 1998, 14, 6941-6947).

The grafting step itself, that is to say the bringing of theorganotrihydrosilane and the silica substrate into contact, is performedin a neutral atmosphere (preferably under argon), using a solution oforganotrihydrosilane in an aprotic solvent. Among the aprotic solvents,it is preferable to use those which have a low hygroscopic character. Byway of example, there may be mentioned carbon tetrachloride,trichloroethylene and toluene.

The organotrihydrosilane may be chosen from the compounds X-E-SiH₃ inwhich E is a spacer segment and X represents H or a reactive terminalfunctional group.

X may be chosen from any functional group capable of allowing theattachment of other organic groups (for example an amine group, ahalogen, an epoxy, a pyridyl, an ester, a tosylate (p-toluenesulfonyl),a heterocumulene (such as an isocyanate, an isothiocyanate or acarbodiimide) or a metal-complexing agent (for example a crown ether, acryptant, a calixarene which is a macrocycle obtained by condensation ofa phenolic derivative with formaldehyde).

The spacer group E makes it possible to confer particular properties tothe film obtained using the process. The group E is chosen from radicalswhich make it possible to obtain an organized monolayer. A radical E ofthe long-chain alkylene type allows interchain interaction. Among theradicals E of the alkylene type, those particularly preferred have from8 to 24 carbon atoms. A radical E comprising two —C≡C— triple bondsallows crosslinking. A radical E comprising a conjugated aromatic chainconfers nonlinear optical properties. By way of example, there may bementioned phenylene-vinylene and phenylene-acetylene radicals. A radicalE of the pyrrole, thiophene or polysilane type confers electronicconduction. A radical E of the heterosubstituted polyaromatic typeconfers photo/electroluminescence properties. By way of example, theremay be mentioned quinones and diazo compounds. A group E of the alkyl orfluoroalkyl type, in particular an alkyl or fluoroalkyl group havingfrom 3 to 24 carbon atoms, makes it possible to use the layers obtainedin chromatography or in electrophoresis.

The organotrihydrosilane solution preferably contains from 10⁻³ to 10⁻¹mole/1. Solutions in which the organotrihydroxilane concentration is ofthe order of 10⁻² mole/1 are particularly preferred.

The duration of grafting is preferably between 4 and 24 hours. Aduration of the order of 12 h makes it possible to obtain good results.

During grafting, the reaction medium should be kept at a temperature ofless than 30° C. The maximum value depends on the substituent X-E-. Thismaximum value tends to decrease when the number of carbon atoms of thesubstituent decreases. The determination of the maximum value for agiven substituent is within the capability of persons skilled in theart. Useful information may be found in particular in Brzoska et al.,(Langmuir, 1994, 10, 4367), which mentions the existence of a criticaltemperature Tc controlling the quality of the self-assembled monolayersobtained from various alkyltrichlorosilanes. The maximum temperature isgenerally less than 30° C. For example, the temperature should be lessthan 30° C. if R is C₁₈H₃₇ and less than 10° C. if R is C₁₂H₂₅.

It is preferable to carry out the reaction under an inert atmosphere, inorder to avoid pollution of the monolayer with organic compounds.

The use of an organosilane X-E-SiH₃ as coupling agent allows the initialformation of an Si—O—Si bond by direct condensation between the Si—Hfunctional group of the reagent with a silanol Si—OH functional groupcarried by the surface of the substrate. This grafting mode considerablylimits the formation of aggregates, which are damaging to the depositionof a homogeneous layer. The use of X-E-SiH₃ additionally has theadvantage of producing by-products which are easy to remove, namely H₂.There is no risk of finding on the treated substrate anionic entities orprotic compounds inherent to prior art processes using chlorosilanes oralkoxysilanes.

It should also be noted that the process proposed may be carried outwithout using a catalyst, unlike the prior art processes consisting inreacting organosilanes with compounds having an active hydrogen, such asacids, alcohols or thiols (cf. E. Lukevics et al., cited above).

The silica substrate modified according to the process of the presentinvention contains at its surface a monolayer of segments X-E- attachedby a covalent bond Si—O—Si, said layer containing functional groups Xwhich are uniformly distributed on the surface and which are accessible.

The process of the invention consists in depositing an organic monolayeron a surface layer of silica which is initially very hydrophilic, theangle of contact being less than 10°. After grafting, the wettability ofthe surface toward ultrapure water greatly depends on the nature of thegroups X-E- of the silane used to form the layer. In the case ofalkylsilanes (E being a linear alkylene), the hydrophobic character ofthe surface results in an angle of contact θ_(H20)≈95-100°. When E is anaryl, the presence of aromatic groups reduces the hydrophobic characterof the surface, which results in an angle of contact θ_(H20)≈69-77°.FIG. 1 illustrates the state of a drop of water on a hydrophilicsurface, the angle θ being less than 90°. FIG. 2 illustrates the stateof a drop of water on a hydrophobic surface, the angle θ being greaterthan 90°.

The images obtained by AFM (atomic force microscopy) show that thesurface is homogeneous and has a very low mean surface roughness (MSR),generally of less than 0.2 nm. The roughness of the treated substrate isindependent of the nature of the organic group grafted, it remains veryclose to that of the untreated initial substrate.

The thickness of the layer obtained is determined by ellipsometry(taking n=1.45 as the value of the refractive index of the surface film,which is the value generally used). This thickness depends on the lengthof the group X-E- and on its orientation relative to the surface of thesubstrate. The thickness is of the order of 1.7 nm when X-E- isoctadecyl, which corresponds to a dispersed layer occupying≅70% of thesurface of the substrate.

The substrate coated with a monolayer obtained by the proposed processis characterized in general by a good covering rate and a goodorganization of the chains at its surface.

In a substrate modified using a silane of the alkyl-SiH₃ type, thecovalent bond through which the substrate is attached to the organicgroup is of the —SiH₂O—Si— type. The presence of SiH₂ groups is revealedby the vibration band {square root}Si—H at 2150 cm⁻¹. This band is notobserved on the substrates modified according to the prior art processeswith the aid of an alkyltrichlorosilane or an alkyltrialkoxysilanecomprising the same alkyl group.

The present invention is described in greater detail with the aid of thefollowing examples, to which it is, however, not limited.

EXAMPLE 1

A series of silicon substrates coated with an organic layer wereprepared by treatment with octadecyltrihydrosilane.

As substrate, silicon (100) disks cut in order to obtain 1×2 cm²rectangular platelets were used.

In a first stage, each platelet was immersed in a solution ofconcentrated HF for a few seconds, until the surface became completelyhydrophobic. Next, each platelet was rinsed with ultrapure water, andthen treated with ozone under UV.

Each platelet thus treated was immediately introduced into a Schlencktube containing 20 ml of a 10⁻²M solution of octadecyltrihydrosilane inCCl₄, and kept in the tube for 24 h at a temperature of 15° C., withoutstirring. After 24 h, the platelets were extracted from the Schlencktubes, washed with CCl₄, with absolute ethanol, and then withchloroform, each washing being carried out under ultrasound, for aperiod of the order of 5 min.

The platelets thus obtained may be stored in an ambient atmosphere,without undergoing degradation.

The angle of contact at the surface of the platelets, measured by thedrop method at equilibrium, is 98°±2, which indicates a hydrophobic andhomogeneous surface.

Under the same conditions as above, silicon platelets were treated withthe aid of octadecyltrichlorosilane, for comparison.

Analysis by infrared spectroscopy in attenuated total reflection (ATR)mode of the surfaces treated with octadecyltrihydrosilane and of thesurfaces treated with octadecyltrichlorosilane gave the results groupedtogether in the following table. C₁₈H₃₇SiH₃ C₁₈H₃₇SiH₃ C₁₈H₃₇SiCl₃C₁₈H₃₇SiCl₃ solution grafted grafted solution {square root}_(as)CH₃(cm⁻¹) 2958 2959 2959 2958 {square root}_(s)CH₃ (cm⁻¹) 2872 2873 28742872 {square root}_(as)CH₂ (cm⁻¹) 2927 2922 2918 2927 {squareroot}_(s)CH₂ (cm⁻¹) 2855 2850 2850 2855 {square root}Si—H (cm⁻¹) 21482150 — —

The substrates treated according to the invention have a band {squareroot}Si—H at 2150 cm⁻¹ which does not exist for the substrates obtainedfrom C₁₈H₃₇SiCl₃ and which corresponds to the existence of Si—H bonds inan environment of the R—SiH₂—O type at the surface of the substrate.

The other bands obtained show that the organization ofoctadecyltrihydrosilane at the surface is a compromise between acomplete crosslinking obtained for octadecyltrichlorosilane grafted andthe absence of organization observed for octadecyltrihydrosilane and foroctadecyltrichlorosilane in solution.

The images obtained by AFM for the platelets of the invention show ahomogeneous surface with a very low roughness, of the order of 0.15-0.20nm.

The thickness of the layers obtained according to the process of theinvention was determined by ellipsometry, taking n=1.45 as the value ofthe refractive index. This thickness is of the order of 1.7 nm, whichcorresponds to a dispersed layer occupying 70% of the surface of thesubstrate.

EXAMPLE 2

The procedure of example 1 was repeated using octadecyltrihydrosilane,changing only the reaction temperature in the Schlenck tube. Two seriesof trials were performed at 5° C. and at 20° C., respectively. Theanalyses carried out on the platelets gave identical results.

EXAMPLE 3

The procedure of example 1 was repeated, but replacingoctadecyltrihydrosilane with phenyltrihydrosilane, all the otherconditions being identical.

The angle of contact measured at the surface of the modified plateletsis 74°±4.

The images obtained by AFM for the platelets of the invention show ahomogeneous surface with a very low roughness, of the order of 0.2 nm.

The thickness of the layers obtained according to the process of theinvention was determined by ellipsometry, taking n=1.45 as the value ofthe refractive index. This thickness is of the order of 0.8 nm, whichcorresponds to a monolayer of high density.

EXAMPLE 4

A series of platelets were treated according to the procedure of example1, but replacing octadecyltrihydrosilane withp-methylstilbenyltrihydrosilane, all the other conditions beingidentical. FIG. 3 illustrates the state of the surface of the plateletafter grafting of the p-methylstilbenyltrihydrosilane.

The angle of contact measured at the surface of the modified plateletsis 85°±3.

The images obtained by AFM for the platelets show a homogeneous surfacewith a very low roughness, of the order of 0.2 nm.

The thickness of the layers obtained was determined by ellipsometry,taking n=1.619 as the value of the refractive index. This thickness isof the order of 19 nm, which corresponds to a monolayer of high density.

EXAMPLE 5

A series of platelets were treated according to the procedure of example1, but replacing octadecyltrihydrosilane with vinylphenyltrihydrosilane,all the other conditions being identical.

The angle of contact measured at the surface of the modified plateletsis 75°±4.

The images obtained by AFM for the platelets show a homogeneous surfacewith a very low roughness, of the order of 0.2 nm.

The thickness of the layers obtained was determined by ellipsometry,taking n=1.546 as the value of the refractive index. This thickness isof the order of 11 nm, which corresponds to a monolayer of high density.

Each platelet thus treated was placed in a 25 ml flask surmounted by acondenser and containing 1 mmol of p-bromotoluene, 9 mg (0.04 mmol) ofpalladium diacetate, 46 mg (0.15 mmol) of triorthotolylphosphine, 2 mlof triethylamine and 10 ml of toluene, the whole under an inertatmosphere. The reaction mixture was heated to 110° C. with gentlemagnetic stirring overnight. After returning to room temperature, eachplatelet was taken out of the flask, and then carefully rinsed withtoluene and with pentane under ultrasound. FIG. 4 illustrates the stateof the surface of the platelet after post-grafting reaction ofp-bromotoluene.

The platelets thus obtained may be stored under an ambient atmosphere,without undergoing degradation.

Analyses carried out on the platelets gave results identical to thoseobtained for the analyses of the platelets treated in example 4.

EXAMPLE 6

The process according to the invention was carried out for a silicasubstrate in the form of colloidal silica.

The substrate is an activated silica marketed by the company Merck underthe name Merck 60F silica.

0.5 g of the activated silica was treated with 1 g ofoctadecyltrihydrosilane in 20 ml of CCl₄ at 19-20° C. for 24 h, withmagnetic stirring. The powder obtained was filtered, washed twice with20 ml of CCl₄, and then 4 times with 20 ml of THF in order to remove anysilanes physisorbed.

It is observed that grains of the powder obtained, when deposited at thesurface of ultrapure water, remain at the surface after 48 hours, whichdemonstrates a perfectly hydrophobic character.

The presence of grafted silane is characterized by infrared spectroscopyand NMR. An IR band at 2165 cm⁻¹ and a signal at −31 ppm in ²⁹Si NMRshow the presence of —O—SiR(H)—O— functional groups. This resultpresupposes the hydrolysis of an Si—H bond, following the attachment ofthe organosilane to the surface.

1. A process for the production of an inorganic substrate which issurface-modified by an organic layer, characterized in that it consistsin bringing an inorganic substrate carrying silanol functional groups atits surface into contact with a solution of an organotrihydrosilane inan organic solvent, at a temperature of less than 30° C.
 2. The processas claimed in claim 1, characterized in that the inorganic substratecarrying silanol functional groups at its surface is a substrateconsisting of silica.
 3. The process as claimed in claim 1,characterized in that the inorganic substrate carrying silanolfunctional groups at its surface is a silicon substrate carrying asilica layer at its surface.
 4. The process as claimed in claim 1,characterized in that the inorganic substrate carrying silanolfunctional groups at its surface is a glass, mica or quartz plate. 5.The process as claimed in claim 1, characterized in that the reaction iscarried out in a neutral atmosphere.
 6. The process as claimed in claim1, characterized in that the solvent is an aprotic solvent.
 7. Theprocess as claimed in claim 6, characterized in that the solvent ischosen from carbon tetrachloride, trichloroethylene and toluene.
 8. Theprocess as claimed in claim 1, characterized in that theorganotrihydrosilane corresponds to the formula X-E-SiH₃ in which E is aspacer segment and X represents H or a reactive terminal functionalgroup.
 9. The process as claimed in claim 8, characterized in that Xrepresents an amino group, a halogen, an epoxy, a pyridyl, an ester, atosylate or a heterocumulene.
 10. The process as claimed in claim 8,characterized in that X represents a metal-complexing agent.
 11. Theprocess as claimed in claim 10, characterized in that X is a crownether, a cryptant or a calixarene.
 12. The process as claimed in claim8, characterized in that the spacer group E is a long-chain alkyleneradical.
 13. The process as claimed in claim 8, characterized in thatthe spacer group E is a hydrocarbon radical comprising two —C≡C— triplebonds.
 14. The process as claimed in claim 8, characterized in that thespacer group E comprises a conjugated aromatic chain.
 15. The process asclaimed in claim 8, characterized in that the spacer group E is apyrrole, or thiophene.
 16. The process as claimed in claim 1,characterized in that the organotrihydrosilane solution contains from0.001 to 0.1 mole/1.
 17. The process as claimed in claim 1,characterized in that the duration of grafting is between 4 and 24hours.
 18. An inorganic substrate coated with an organic monolayer,obtained by the process as claimed in any one of claims 1 to
 17. 19. Aninorganic substrate coated with an organic monolayer obtained by theprocess as claimed in claim 12, characterized in that the monolayerconsists of alkylene radicals attached by —Si—H₂O—Si— bonds in which theSiH₂ groups are characterized by a vibration band {square root}Si—H at2150 cm⁻¹.